Your brother's GL has the same OM642 engine as my wife's ML320 CDI

On the glow plugs, they superseded the glow plug design and they have a different operating voltage than the originals. So you have to replace all 6 and the control module, which is right up front on the passenger side of the engine. Look for the big fat red wire coming from the under hood fuse box on the passenger side, and trace it to the controller hidden under a bracket on the air mixing chamber.

Check these links I wrote:

OM642 service tips

OM 642 camshaft position sensor replacement

OM642 glow plug replacement

W164 front light bulb replacement

W164 battery replacement

And I didn't start this thread but I've been through it, my posts start a few pages in.

A journey into the the infamous Om642 oil cooler leak!

__________________
The OM 642/722.9 powered family
Still going strong
2014 ML350 Bluetec (wife's DD)
2013 E350 Bluetec (my DD)

both my kids cars went to junkyard in 2023
2008 ML320 CDI (Older son’s DD) fatal transmission failure, water soaked/fried rear SAM, numerous other issues, just too far gone to save (165k miles)
2008 E320 Bluetec (Younger son's DD) injector failed open and diluted oil with diesel, spun main bearings (240k miles)

1998 E300DT sold to TimFreeh
1987 300TD sold to vstech